It is essential in telephone line-interface circuits that the balancing resistors (telephone line interface resistors), in combination with the line cards (circuit boards), meet three levels of adverse conditions. Such levels are required by telephone companies of their suppliers of telephone switching equipment which uses these balancing resistors on line cards.
A first-level condition is one where the resistors and the line card survive and continue to function despite certain occurrences (conditions). These occurrences include lightning transients, and also include some types of improper connections such as could be made by technicians.
A second-level condition is one where, if there is no protection in the design, the balancing resistors overheat, the overheating being so significant as to cause melting and/or burning of resin-cased components on the line card, and/or cause burning of the line card (circuit board). Occurrences that give rise to a second-level condition are other types of improper connections which could be made by technicians.
A third-level condition is one where a sudden high-voltage surge is applied to the telephone system. Typically, an occurrence creating a third-level condition, if there is no protection in the design, is where power lines fall on or otherwise contact telephone wires in the system. Third-level conditions are often such as to melt small wires that are in the telephone system not only in the cages for the line cards, but also in regions outside of and remote from such cages. The small wires may not only melt or be otherwise damaged, but may burn and/or may cause a fire in the premises where the wires are located.
It is important for the telephone companies to have balancing resistors that meet and provide protection as necessary for the three levels of adverse conditions.
It is not enough that some of the balancing resistors in a production run, in combination with the line cards on which they are mounted, meet the three levels of adverse conditions. They must meet these levels reliably. Thus, if only some of the resistors meet the levels, that will not suffice; substantially all of the resistors must meet them or there is no reliability.
To summarize, therefore, not only must the balancing resistors perform the electrical functions that are required by telephone companies; they must reliably meet the three levels. That, however, is not all that the balancing resistors must do if their manufacturer is to survive and prosper in the balancing resistor business.
For example, it is often demanded that balancing resistors have low resistance values. Low-value resistors are vastly more susceptible to overheating than are high-value ones, simply because of the laws of physics. As another example, it is more and more demanded that balancing resistors be physically small in size, but such small-size resistors radiate less heat and thus are far more likely to fail heating tests. As a third example, some telephone companies want--at least for some applications--the balancing resistors and associated line cards to reset automatically after having open-circuited in response to an incipient second-level condition. In other words, they want everything to resume operating, after a second-level condition, just as if that condition had never happened, and without any need for human intervention.